There is another new field of semiconductor lighting technology, Solid State Lighting, developed upon the aforementioned two technology fields. A novel light source based on In—Ga—N heterojunction (please refer to S. Nakamura, Blue laser, Springer Verlag, Berlin, 1997.) has been under development. The In—Ga—N heterojunction containing a large amount of quantum wells has been developed first by a Japanese scholar, Shuji Nakamura.
The first batch of luminescent heterojunction containing a large amount of quantum wells on its luminescent planes was synthesized in 1994. Five years later, the white light-emitting diode (LED) was granted patent right. The patented LED is a semiconductor heterostructure containing phosphor powder (please refer to ROC Patent No. TW156177 granted to S. Schinuzu et al., Jan. 1, 2005.). The blue light emitted from the heterojunction (i.e. P-N junction) is combined with yellow light emitted from the phosphor powder to form white light.
In as early as 1960s, Bell Lab created LEDs employing GaAs—GaP series elements with emissions at 800˜900 nm as heterojunction and the Anti-Stokes phosphor powder (Y2O3S:ErYb) as luminescent material to convert light of the infra-red region into visible light. For many years, LEDs have been developed using the same structure, in which visible light is emitted from the red and green sub-energy bands of visible light. If the radiation from the first layer, the phosphor powder, of the “Double-layer” structure (i.e. phosphor powder layer and heterojunction layer) is a short-wavelength radiation, the structure proposed by Russian engineers, B. C. Ablamov and B. P. Sushikov (please refer to A. King. “Semiconductor”, World Publishing Company, Moscow, 1982) falls into the category of GaN structure, upon which is covered with the Stokes phosphor powder (converting part of primary radiation from the GaN heterojunction structure into long-wavelength radiation). In summary, the LEDs' Double-Layer Structure has been widely known in 1965-80.
White LEDs are characterized by that it combines two radiations: short-wavelength radiation (blue light) and long-wavelength radiation (yellow); the combination is based on Newton's principle of complementary color. According to the principle, a pair of complementary colors, blue and yellow, pale blue and orange-yellow, blue-green and red, for example, can be combined to form white light. For a long time, the electron-emission technology such as cathode ray tube (CRT) of black-and-white television as well as Radar has employed the principle of complementary color (please refer to K. Mordon and B. Zvoryki, Television, the World Publishing Company, 1955; Leverenz Luminescence of solid, NY, 1950). Fluorescent particles of ZnS and Ag are distributed on the fluorescent screen in Radar tubes to generate photoluminescence. The landing spots of electron beams are shown to have very bright white light, which is formed by combining two individual spectrums of the phosphor power. On the other hand, the display screen of black-and-white TV employs multi- or single-layer of phosphor powder, which comprises two cathodes phosphor powders (of blue and yellow) emitting white light by combining blue and yellow lights.
Consequently, the aforementioned approach is to convert a short-wavelength radiation into a long-wavelength one by a phosphor powder. Two lights (blue and yellow) emitting together to form white light. This approach has been known for a long time.
Later, there emerged a yellow phosphor powder with special composition, which is applied along with the In—Ga—N heterojunction with a blue light radiation. Designers have proposed to use yttrium-aluminum garnet Y3Al5O12:Ce as this yellow phosphor powder (please refer to ROC Patent No. TW156177 granted to S. Schinuzu et al., Jan. 1, 2005.). Therefore, many companies producing white LEDs are forbidden to use this phosphor powder because these companies are not patented to use the material. However, the cathode phosphor powder based on Y3Al5O12:Ce was developed by Blasse G in 1960s (please refer to B. C. Ablamov B. P. Sushikov, Soviet Union Publication, N635813, Sep. 12, 1977) and widely applied in actual production to make scintillator, especially CRT screens. In a CRT screen, the mixture of blue and yellow cathode phosphor powders, Y2SiO5:Ce and (Y,Gd)3Al5O12:Ce, are used.
Although the technology has been widely applied, some drawbacks still remain: (1) LEDs can only generate white light with a high color temperature T≧6500K and chromaticity coordinates as 0.30≦x≦0.31 and 0.30≦y≦0.32; (2) the first generation of LEDs has a very low efficiency, less than 10 lumen/watt.
Presently, there are a large number of patents granted to improve the aforementioned drawbacks. The improvements are largely related to the creation of so-called warm white light source with a color temperature 2500K≦T≦3500K and chromaticity coordinates as 0.40≦x≦0.44 and 0.38≦y≦0.44.
Another plausible scheme concerning the creation of such a warm-white light source is to employ phosphor powder which can generate orange-yellow or orange-red under the blue light activation of the In—Ga—N heterojunction. The present inventors have delivered the concept in the US Patent Application No. US2007/0272 899A (Please refer the US Patent Application No. US2007/0272 899A submitted by N. P. Soshchin et al., Nov. 29, 2007); a phosphor powder of this kind was proposed and can generate orange-red radiation under the activation of cerium and praseodymium ions of yttrium garnet.
This kind of phosphor powder has been produced in many companies across the world and can ensure that (1) the color temperature of the white semiconductor light source is T>3000K, in particular, 3200˜3500K, and (2) the luminescence efficiency of LEDs can reach 50˜75 lumen/watt.
Nevertheless, there exist some substantive drawbacks for this phosphor powder: (1) the non-uniform white color of λmax=548 nm, the peak wavelength of the radiation of the phosphor powder on the lens cover of LEDs, and the peak wavelength λmax=610 nm, leads to a non-uniform white light; (2) the synthesis of the phosphor powder is complicated and hard to be repeated; (3) the quantum output of the phosphor powder produced is low, 75˜85% only; and (4) white light with a color temperature T≦3000K cannot be obtained. Thus, there exists a need to overcome these drawbacks.